The concentration of atmospheric carbon dioxide (CO2) continues to rise, as shown by, for example, IPCC, Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007 [Core Writing Team, Pachauri, R. K and Reisinger, A. (eds.)], IPCC, Geneva, Switzerland, 104 pp. It is becoming increasingly imperative to invent efficient and cost-effective technologies for controlling the atmospheric CO2 concentration. The concentration of atmospheric carbon dioxide (CO2) is rising at the rate of approximately 2 parts per million per year (ppm/yr). The challenge of reducing the concentration of atmospheric CO2 represents an opportunity to invent new, cost-effective technologies to solve this problem.
Techniques for removing CO2 from streams of mixed gases, such as removing the CO2 from power-plant flue-gas emissions or removing CO2 from the atmosphere, typically involve a two-step process of capture and regeneration. First, the gas is contacted with an aqueous “pre-capture solution” that reacts with the CO2 gas in the mixed-gas stream, “capturing” the CO2 into what is then referred to as a “post-capture solution.” Next, a stream of pure CO2 gas is regenerated from this CO2-rich aqueous post-capture solution. Various pre-capture solutions exist, with different solutions being preferred depending on the concentration of CO2 in the mixed gas source. For mixed gas streams with low concentrations of CO2—such as the atmosphere with a CO2 concentration of 386 ppm as of 2009 as shown by Dr. Pieter Tans, NOAA/ESRL—aqueous hydroxide pre-capture solutions such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), aqueous carbonate pre-capture solutions such as potassium carbonate (K2CO3) or sodium carbonate (Na2CO3), or aqueous bicarbonate pre-capture solutions such as potassium bicarbonate (KHCO3) or sodium bicarbonate (NaHCO3) are likely candidates for CO2 pre-capture solutions. Other pre-capture solutions are known, for example, monoethanolamine (MEA), which is used in gas stream scrubbing applications to remove, for example, CO2 from flue gas. The capture of CO2 gas into these pre-capture solutions converts the original hydroxide/carbonate/bicarbonate pre-capture solutions into a more acidic post-capture solution consisting of a mixture of hydroxide (KOH or NaOH), carbonate (K2CO3 or Na2CO3), and/or potassium bicarbonate (KHCO3) or sodium bicarbonate (NaHCO3) post-capture solutions, as examples.
Once the CO2 gas is captured from the mixed-gas stream into the pre-capture solutions in the ionic forms CO3(2−) and/or HCO3− to form the post-capture solutions, pure CO2 gas is typically regenerated from the solution. The overall effect of this process of capture and regeneration is the separation and concentration of CO2 gas from a pre-separation mixed-gas stream with a relatively low mole fraction of CO2 gas into a post-separation gas stream that possesses a higher mole fraction of CO2 gas than the pre-separation stream. Under the right conditions, the mole fraction of CO2 in the post-separation stream may be unity, that is, the post-separation stream may be a pure stream of CO2 gas. After capture and regeneration, the post-separation gas can then be, for example, geologically sequestered, or incorporated into useful products such as concrete, as shown by Calera, Green Cement for a Blue Planet, http://www.calera.com/index.php/technology/technology_vision/ (last visited Sep. 9, 2010); plastics, as shown by G. A. Olah et al., Beyond Oil and Gas: The Methanol Economy, Wiley-VCH (2006); or liquid hydrocarbon fuels, as shown by F. S. Zeman & D. W. Keith, Carbon Neutral Hydrocarbons, Phil. Trans. R. Soc. A, 366, 3901-3918 (2008), and PARC, Renewable Liquid Fuels, (last visited Sep. 9, 2010). Many of the possible uses of the regenerated CO2, such as sequestration or reaction to liquid fuels, for example, require the pressurization of the CO2 to pressures greater than 1 atm.
Bipolar membrane electrodialysis (BPMED) can be used to convert aqueous salt solution into acids and bases without the addition of other chemicals. A component of BPMED devices is ion exchange membranes used to separate ionic species in solution when an electrical field is applied across the membranes. Performing BPMED on certain solutions may create gas bubbles adjacent to the membrane surface that can block ion transport and reduce the effective membrane surface area, causing increased cell resistance and localized “hot spots” of very high current density that lead to shortened membrane lifetimes. As a result, commonly used input and output solutions are selected so that they do not evolve significant quantities of gas inside the membrane stack at ambient pressure, which excludes an entire class of gas-evolving solutions from electrodialytic treatment. Example embodiments address these and other disadvantages of the conventional art.